The present invention relates to a logic circuit and, more particularly, to a logic circuit capable of selection of logical functions.
Along with the progress of integrated circuit (IC) manufacturing technology in recent years, increasingly higher density has been achieved for IC's. The area S of an IC is equal to (A.times.B), where A and B represent its length and width, respectively, while the circumferential length L of the IC is equal to .hoarfrost.2.times.(A +B)}. A denser IC means that a greater number of circuit elements can be formed in a given area S and accordingly a greater variety of functions can be performed. However, the number of input/output (I/O) terminals that can be arranged on the circumference of the IC to be electrically connected to external circuits is limited by the length L because the terminals cannot be made infinitely minute, resulting in a tendency of the number of terminals to become insufficient. There are cases, for instance, wherein a first circuit and a second circuit, of whose functions most are common but only some are different, are formed in a single IC to reduce their production cost, and the first and second circuits are discriminately used according to the purpose of use. In such an instance, not all of the I/O terminals used by the first circuit are used by the second circuit, so that virtually useless I/O terminals are provided on the IC. This is undesirable in view of the above-mentioned tendency of the number of terminals to become insufficient.